The present invention relates to an optical fiber strain and temperature measurement apparatus using Brillouin scattered light.
With the evolution of optical fiber communications, distributed optical fiber sensing, in which the optical fiber itself serves as the sensing medium, has become an active area of research. Representative distributed optical fiber sensing is optical time domain reflectometry (OTDR), in which optical pulses are launched into an optical fiber from one end of the optical fiber, and light backscattered within the optical fiber is measured with respect to time. Backscattering in an optical fiber includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Among others, OTDR that measures spontaneous Brillouin scattering is referred to as Brillouin OTDR (BOTDR) (see, for example, K. Koizumi, et al., “High-Speed Distributed Strain Measurement using Brillouin Optical Time-Domain Reflectometry Based-on Self-Delayed Heterodyne Detection”, ECOC2015, P.1.07, September 2015).
Brillouin scattering can be observed at frequencies frequency-shifted on Stokes and anti-Stokes side with the frequency shift of the order of GHz with respect to the center frequency of the optical pulse launched into the optical fiber. The spectrum of Brillouin scattering is referred to as the Brillouin gain spectrum (BGS). The frequency shift and the spectral line width of the BGS are referred to as the Brillouin frequency shift (BFS) and the Brillouin line width, respectively. The BFS and the Brillouin line width vary depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of silica-based standard single-mode optical fiber, it is reported that, for an incident wavelength of 1.55 μm, the frequency shift amounts of the BFS and the Brillouin line width are approximately 11 GHz and approximately 30 MHz, respectively. Also, it is known that the frequency shift amounts of the BFS associated with strain and a temperature change inside a single-mode optical fiber are 0.049 MHz/με and 1.0 MHz/° C., respectively, for an incident wavelength of 1.55 μm.
Here, since the BFS has dependencies on strain and temperature, BOTDR has been attracting attention for the purpose of monitoring large constructions represented by bridges and tunnels, potential areas of landslide occurrence, or the like.
BOTDR generally performs heterodyne detection to measure spectrum waveform of spontaneous Brillouin scattered light arising in an optical fiber with the use of reference light prepared separately. The intensity of spontaneous Brillouin scattered light is lower than the intensity of Rayleigh scattering light by two through three orders of magnitude. Thus, heterodyne detection is useful in increasing the minimum light receiving sensitivity.